On Thu, Sep 07, 2017 at 09:28:48AM +0200, Peter Zijlstra wrote: > On Thu, Sep 07, 2017 at 11:34:12AM +0530, Prateek Sood wrote: > > Remove circular dependency deadlock in a scenario where hotplug of CPU is > > being done while there is updation in cgroup and cpuset triggered from > > userspace. > > > > Example scenario: > > kworker/0:0 => kthreadd => init:729 => init:1 => kworker/0:0 > > > > kworker/0:0 - percpu_down_write(&cpu_hotplug_lock) [held] > > flush(work) [no high prio workqueue available on CPU] > > wait_for_completion() Hi Prateek, so this is: _cpu_down(): cpus_write_lock(); // percpu_down_write(&cpu_hotlug_lock) cpuhp_invoke_callbacks(): workqueue_offine_cpu(): wq_update_unbound_numa(): alloc_unbound_pool(): get_unbound_pool(): create_worker(): kthread_create_on_node(): wake_up_process(kthreadd_task); wait_for_completion(); // create->done , right? Wonder running in a kworker is necessary to trigger this, I mean running a cpu_down() in a normal process context could also trigger this, no? Just ask out of curiosity. Regards, Boqun > > > > kthreadd - percpu_down_read(cgroup_threadgroup_rwsem) [waiting] > > > > init:729 - percpu_down_write(cgroup_threadgroup_rwsem) [held] > > lock(cpuset_mutex) [waiting] > > > > init:1 - lock(cpuset_mutex) [held] > > percpu_down_read(&cpu_hotplug_lock) [waiting] > > That's both unreadable and useless :/ You want to tell what code paths > that were, not which random tasks happened to run them. > > [...]
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